1. Field of the Invention
This invention relates to a straight or extension ladder comprising anti-slide-out means for determining the minimum ladder set-up angle whereby the base of the ladder is precluded from sliding away from the wall or other structure against which the ladder is leaning upon application of a weight on the ladder.
2. Description of Prior Art
A straight or extension ladder maintains its equilibrium when placed against a wall or other structure by the friction resistance against sliding that is created between the side rail feet and the ground surface. When this friction force is not sufficient, the base of the ladder slides away from the wall dropping its climber. Over one-third of all ladder accidents are caused by ladder slide-out.
The equations of equilibrium for straight or extension ladders indicate that the resistance against slide-out increases with the steepness of the ladder. The steepness of the ladder is normally characterized by the acute angle formed between the ground surface and the center line of the ladder. In the United States, ladders are designed and tested using an angle of 75.52xc2x0, which is also used as the limiting ladder set-up angle to avoid slide-out. The safety factor against ladder slide-out falls off very quickly as the ladder angle becomes shallower.
There are a number of popular techniques for establishing the 75.52xc2x0 ladder angle. The first of these is the one-in-four method by which the angle is set by arranging the geometry such that the base-to-wall distance is one-forth of the active ladder length.
Another method involves the mounting of an xe2x80x9cLxe2x80x9d on the side rail of the ladder in a special orientation. When the ladder is correctly set up, the L achieves a natural orientation with its legs in a vertical and horizontal direction.
Yet a third method involves anthropometric set-up in which four instructional steps are placed on ladder labels to achieve a ladder angle of approximately 75xc2x0. These instructional steps arexe2x80x941) place toes against bottom of ladder side rails; 2) stand erect; 3) extend arms straight out; and 4) palms of hands should touch top of rung at shoulder level.
A further means for achieving proper set-up of a ladder is taught by U. S. Pat. No. 2,845,719 wherein a bubble level is attached to the outside of the ladder side rail at eye level to disclose any chosen set-up angle. U.S. Pat. No. 3,118,234 teaches a pendulum device attached to the outside of the ladder side rail whereby, when the ladder is set up at a ladder angle of 75xc2x0, a mark on the pendulum housing lines up with the pendulum. If the ladder base is too far in or out, the pendulum housing is marked appropriately xe2x80x9cmove, inxe2x80x9d or xe2x80x9cmove outxe2x80x9d so that the user will move the ladder base in the correct direction.
U.S. Pat. No. 5,740,881 teaches yet another approach in which an electronic circuit alarm are attached to a ladder with two sensors. One of the sensors determines the side-to-side orientation of the ladder while the other determines the ladder inclination angle. When incorrectly set up, the alarm sounds and the actual angles are displayed.
Yet another device for determining proper inclination of a ladder is a xe2x80x9cmonster eyexe2x80x9d, named after a toy, which is mounted under the sixth base section rung at eye level. The monster eye consists of two concentric spheres, the inner sphere of which is opaque and weighted on one side and the outer sphere of which is transparent. Between the spheres, the space is filled with liquid that allows the inner sphere to rotate freely so that its weighted side can remain in a downward-facing orientation. When en equator line on the inner sphere falls between two closely spaced parallel lines painted around the equator of the outer sphere, the ladder has achieved an inclination angle of 75.5xc2x0.
One problem associated with each of the above described methods and devices is that the set-up protocol may be completely ignored by the users, who may adopt any arbitrary inclination angle that suits their immediate fancy, risking thereby a non-safe ladder set-up.
It will also be appreciated that there are numerous devices known in the art for stabilizing a ladder. U.S. Pat. 5,341,899 teaches an anti-skid hand leveling device for ladders which includes a pair of devices consisting of a guide rail along which an upper carriage and a lower carriage slide independently. The upper carriage provides a mounting platform onto which a brace is rotatably mounted. The lower carriage provides a mounting platform onto which an outrigger-type foot is mounted. When pivoted to a specified angle and lowered so as to contact the ground, the brace prevents the ladder from skidding in a direction away from the object against which the ladder is resting. Similar devices are taught by U.S. Pat. No. 4,723,629 and U.S. Pat. No. 4,130,181. See also U.S. Pat. No. 5,918,698; U.S. Pat. No. 4,632,220; U.S. Pat. No. 3,059,723; U.S. Pat. No. 2,868,427; U.S. Pat. No. 1,710,026; U.S. Pat. No. 1,352,566; U.S. Pat. No. 840,365; U.S. Pat. No. 776,446; and U.S. Pat. No. 530,374. Although providing stabilization for straight and extension ladders, none of these prior art references provides any means for ensuring proper set-up of the ladder so as to preclude ladder inclination angles below a specified limiting ladder set-up angle.
Accordingly, it is one object of this invention to provide an apparatus for ensuring proper ladder inclination angles which preclude slide-out of the base of the ladders upon application of a weight to the ladder.
It is another object of this invention to provide a method and apparatus for proper ladder set-up which passively rejects any ladder inclination angle below a specified limiting ladder set-up angle xcex8, for example 75.5xc2x0.
These and other objects of this invention are addressed by a non-self-supporting ladder comprising two substantially parallel, elongated, spaced apart side rails having an upper and a lower end and a plurality of substantially parallel, spaced apart rung elements joining the spaced apart side rails. An inboard roller assembly comprising a bracket and a roller rotatable over its central axis is connected to each of the spaced apart side rails, whereby the central axes of the rollers are oriented so as to be essentially parallel to the spaced apart rung elements joining the spaced apart side rails. The inboard roller assemblies are disposed so as to impose a specified ladder inclination angle xcex8 when the lower end of the spaced apart side rails and the rollers rest on a substantially flat horizontal surface.